A serosa is a type of epithelium embryologically derived from the mesoderm, thus a serosa can also be called a [mesothelium]. Serosal membranes are found forming sacs that surround structures in the body, such as the pericardium, pleural membranes, and peritoneum.

Serous membranes are characterized by a thin single-layer mesothelium covering a layer of vascularized loose connective tissue. A serosa produces (and absorbs) a watery fluid called "serous fluid" which serves to lubricate organs that require movement such as the heart, lungs and digestive tract : pericardial fluid, pleural fluid, and peritoneal fluid.

The excess production (effusion) of serous fluid can cause serious pathology, such as  pericardial tamponade, hydrothorax, or ascitis (peritoneal effusion)